The present invention relates to a method and apparatus to reduce distortion in currents IA, IB and IC feeding an AC/DC rectifier system. This system corresponds to a transformer in series with an AC/DC rectifier.
AC/DC rectifier systems convert alternating current (AC) into direct current (DC) and have become vital elements in industry. They allow the functioning of variable speed AC motors, electrochemical plants, induction furnaces, HVdc energy transmission systems, railway systems, uninterruptible power supplies (UPS), telephone plants, etc.
Regrettably, currents feeding AC/DC rectifier systems are distorted, which in turn produces critical problems in the network. This is a worldwide problem and in Chile a law has recently been passed, based on the North American regulation, which involves fines and disconnections in extreme cases of distortion.
Distorted currents are undesirable as they provoke voltage distortions in the network. These voltage distortions affect other network loads, especially where computer and communication systems are involved. Further problems include overheating of equipment (motors, transformers, neutral conductors, etc), overvoltages owing to resonant conditions in the network, errors in measuring instruments, malfunction of protective devices and interference with both communication and control systems.
At present, the market is offering several alternatives to reduce current distortion. Since this is such a complicated technical problem the technologies offered are numerous, among them are the use of passive and active filters. These elements are connected, either on the primary or secondary side of the transformer feeding the rectifier and can be repaired without disconnecting the system, a characteristic also offered by the proposed apparatus.
Passive and active filters reduce distortion in different ways. Passive filters do so by absorbing the distortion component of the line current. By contrast, active filters inject the distortion component to cancel current distortion along the power line.
The main disadvantage of passive filters is their complicated design, given that it is necessary to know complex parameters of the electric system feeding the AC/DC rectifier. Other disadvantages are related to cost, size, temperature changes, component aging and instability in the presence of system disturbances. Additionally, the filter overloads when other AC/DC rectifiers are in the vicinity and, in the case of a diode rectifier, injects reactive power, which elevates elevating the voltage network.
Similarly, the main disadvantage of active filters is their complexity, which implies bulky control circuits where state-of-the-art microprocessors (DSP) play a fundamental role. Other disadvantages are the need for an energy storing element (L or C) with a critical control circuit and the generation of electromagnetic disturbances into the network.
The proposed apparatus to reduce current distortion is an active filter installed on the DC of the rectifier. It is demonstrated in a laboratory prototype that this apparatus achieves notably low distortion levels (ideally zero), at low cost, with simple control circuits (without DSP) and gives a robust performance with high dynamic response under rapid load variations. It can also be repaired on line without the need to disconnect the system, has no energy storing elements L or C and generates a low level of electromagnetic disturbance into the network. This demonstrates that if the currents on the DC side are accurately shaped it is theoretically possible to achieve currents with zero distortion on the AC side of the rectifier system.